The present invention relates in general to communication systems, and is particularly directed to a new and improved antenna range test mechanism that uses direct spread-spectrum based test signals to mitigate against impairments to an antenna test range caused by multipath and or the presence of one or more interfering emitters.
The design and testing of a radio wave antenna has historically been principally concerned with the antennas performance (especially gain) in the direction of its boresight or main beam axis. For this purpose, as diagrammatically illustrated in FIG. 1, an antenna 10 whose performance is to be measured may be mounted inside a compact test range 12, such as an EMI-shielded anechoic chamber, that is configured to eliminate reflections and interference from unwanted sources of electromagnetic radiation. Testing the antenna typically involves directing radio wave emissions from a test signal source 14 toward the antenna 10, and measuring the amplitude and phase response of the antenna by means of a range receiver 16, the output of which may be displayed or recorded via an associated test and measurement workstation 18. As the relative orthogonal principle planes (e.g., azimuth and elevation) parameters between the antenna 10 and test signal source 14 are varied (for example, by moving either the antenna or the test source), both boresight and off-axis gain parameters are derived.
Unfortunately, at relatively low frequencies, such as UHF, the size of such a test range becomes physically and cost-wise prohibitive, making it necessary to test the antenna design outdoors. While finding a location to set up an outdoor antenna test range that is free of interferers may not have been particularly difficult several decades ago, it has now become a significant problem due to the proliferation of wireless commercial products, such as cellular phones and citizen band radios, as well as specular reflections from buildings and the like.
Moreover, this interference and reflection free test range problem is compounded by the fact that antenna designers are no longer necessarily principally interested in boresight performance; they now must measure the antenna""s off-axis characteristics, in order, for example, to evaluate its ability to place nulls on one or more of the continually growing number of interferers, such as the cellular phone and CB radio devices, referenced above. Thus, the outdoor test range operator could face the dilemma of trying to measure side lobe characteristics of the antenna, without the presence of one or more likely interferers, while at the same time designing the antenna to exhibit a characteristic that allows placement of nulls on such interferers.
In accordance with the present invention the test range impairment problem described above is effectively mitigated by employing a test signal whose characteristics facilitate the signal processing or electronic rejection of all other signals that may be present in the test range, and thereby allows both main beam and sidelobe characteristics of the antenna to be accurately measured. For this purpose, the present invention uses a test signal, which has very high autocorrelation properties with itself on the one hand for test measurement purposes, and high cross-correlation properties with signals other than itself (especially including interferers and specular reflection) for interference rejection. A signal waveform that readily complies with this requirement is a direct sequence spread-spectrum signal.
Pursuant to a non-limiting, but preferred embodiment of the invention, just as in the test ranges of FIGS. 1 and 2, the antenna under test may be mounted at a location at which measurements are to be conducted by range receiver equipment connected to the antenna. To measure antenna gain and phase parameters for variations in orthogonal principle planes, the antenna""s response may be measured as a test range signal source, that is operative to generate a direct spread-spectrum signal, is moved relative to the antenna""s boresight axis. Conversely, the test source may be fixed and the antenna""s pointing angle varied in orthogonal principle planes.
The test range receiver equipment, to which the output of the antenna under test is coupled, may comprise an RF receiver section which demodulates and bandpass filters the spread test signal received from the test signal source and outputs a signal that is despread in a correlation processor to recover the earliest line-of-sight emission from the test source. Multipath is circumvented by selecting the earliest in time (first-to-arrive) correlator output signal which is time-aligned with the reference PN signal, whose energy content exceeds a prescribed threshold in order to identify the line-of-sight traveling test signal of interest.
Impairments due to RF emissions other than those sourced from the spread signal test signal source are avoided, since the energy in the correlator outputs for these other emissions is highly cross-correlated with the reference PN sequence, and therefore effectively null. The energy in the highly autocorrelated output of the correlator processor is digitized and processed by way of the antenna performance measurement algorithm executed by a test processor.
Another advantage of the invention is the converse of the above, i.e., the test range signal is highly cross-correlated to licensed transmitters, and therefore interference of such signals is eliminated or reduced to a non-interfering level.